1. Field of the Invention
The present invention relates to a developing device of the liquid developer circulating type which includes a developing head provide with a rod-like body and at least one liquid developer flowing passage having a slit-shaped opening on the top of the rod-like body, wherein a latent image formed on a recording medium such as an electrostatic recording paper which is fed on the slit-shaped opening, can be developed into a visible image by a liquid developer flowing through the passage.
More particularly, it relates to a developing device of the liquid developer circulating type wherein a liquid developer in a tank is introduced into a developing head by the action of negative pressure created by a suction pump to develop a latent image on a recording medium, and then returned into the tank after this development of the latent image is finished.
2. Description of the Related Art
The developing device of the liquid developer circulating type intended to use negative pressure suction force created by the suction pump is conventionally well known. The developing device of this type includes a developing head provided with liquid developer flowing passages each having a slit-shaped opening on the top of a rod-like body to allow liquid developer to act on a latent image on a recording medium, a tank for storing the liquid developer supplied to the developing head, a suction pump for introducing the liquid developer in the tank into the liquid developer flowing passages, which are provided with the slit-shaped openings, by suction force created by negative pressure, and a piping for connecting these three components to one another.
The operation of the developing device is as follows. The slit-shaped openings of the liquid developer flowing passages in the developing head are covered by the recording medium itself and the suction pump is made operative while moving the recording medium relative to the developing head. The whole of that part of the liquid developer circulating line which is located on the upstream side of the suction pump and which includes the liquid developer flowing passages are thus made negative in pressure and the liquid developer in the tank is sucked into the developing head by suction force created by this negative pressure in the part of the liquid developer circulating line. The liquid developer thus sucked flows through the liquid developer flowing passages having the slit-shaped openings to thereby develop the latent image on the recording medium by the liquid developer. After this development of the latent image, the liquid developer is returned into the tank.
In the case of this developing device of the liquid developer type having the above-described arrangement, a predetermined negative pressure must be held in the liquid developer circulating line extending from the tank to the suction pump at least at the time of development. It is therefore needed that the slit-shaped openings of the liquid developer flowing passages in the developing head are air-tightly covered by the recording medium. When the recording medium is wrinkled and slackened to damage its air-tight covering relative to the slit-shaped openings of the passages and when air is leaked through the piping, the liquid developer cannot be circulated through the line to develop the latent image on the recording medium as desired. Further, when such a recording medium that is high in surface roughness is used, same demerit is caused
In order to eliminate this demerit, a device provided with a means for detecting any abnormal level of negative pressure to sound alarm was proposed. The Japanese Utility Model Publication Sho 52-25153, for example, disclosed a means for detecting any abnormal level of negative pressure by a negative pressure sensor. According to this means, the level of negative pressure is detected by the negative pressure sensor, it is asked whether the value thus detected is larger or smaller than a reference value previously set, and alarm is sounded or displayed depending upon the result thus obtained.
In the case of the developing device of the liquid developer circulating type intended to use suction force created by negative pressure as described above, the pumping capacity of the suction pump is fixed to a certain level. When the recording media are not uniformly stuck relative to the top of the developing head, therefore, the suction force created by negative pressure changes accordingly. Particularly when they are wrinkled and slackened, stable circulation of the liquid developer is damaged not to apply excellent development relative to the latent images on them. Further, in the case of the conventional developing device in which the pumping capacity of the suction pump is fixed to a certain level, a relatively long time is needed until the liquid developer can start its stable circulation through the liquid developer circulating line after the start of the suction pump. It is therefore after the time needed to rise the suction pump lapses that the developing process can be started. This asks a long time before the developing process is practically started. Still further, the suction pump is started while carrying the recording medium to more reliably stick the recording medium onto the developing head in the case of the conventional developing device. As the result, large quantity recording medium is wasted.
It is imagined that the pumping capacity of the suction pump is made high enough to eliminate the above-mentioned drawbacks. When this is realized, the recording medium can be reliably stuck onto the developing head and the time needed before the developing process is practically started can be shortened. In addition, the large quantity recording medium which are wasted can be reduced.
However, the running load of the recording medium is increased because the suction force by which the recording medium is stuck onto the developing head is made large. Force for feeding the recording medium is needed to become large. This causes the medium carrying motor and the drive circuit to be made large in size and capacity.
Further, the surface unevenness of the recording medium is crushed and deformed because the image-formed surface of the recording medium is strongly pressed against the top of the developing head. This causes the surface roughness of the recording medium to be changed and the fine amount of air leaked through the developing head is changed accordingly. As the result, the running load of the recording medium is further increased and the developing characteristics of the device are changed accordingly. Still further, in the case where the device in which the above-mentioned developing means is incorporated is intended to record an color image on the recording medium by overlapping plural colors one upon the others, undesirable influences are added to developing and recording characteristics of the device. Namely, it becomes difficult to hold correct the recording characteristics which depend upon the surface roughness of the recording medium. As the result, the quality of images recorded is deteriorated. Still further, it sometimes happens that pieces of dust stick to the surface of the recording medium and that these pieces of dust are press-extended. This causes images recorded on the surface of the recording medium to seem humble.
The common paper, synthetic paper or transparent plastic film is used as the base sheet of those recording media which are used by the above-mentioned multi-colored images recording device. When the base sheet for a recording medium is different from the one for another recording medium, the manners of processing recording layers coated on the image-recorded surface of these two recording media are different from each other. If these manners of processing the recording layers are different, the surface roughness of one of the recording media becomes different from that of the other.
The recording media which are different in surface roughness from the other cause the amount of air leaked through the gap between the slit-shaped openings of the liquid developer flowing passages on the developing head and the recording media to be changed. Even when the recording media are neither wrinkled nor slackened, therefore, the level of negative pressure in the liquid developer circulating line and the amount of the liquid developer circulated therethrough change depending upon different surface roughnesses of the recording media. When the amount of the liquid developer circulated changes, the developing capacity of the device becomes different to thereby form images short in density or cause base fog because of excessive development.